Slip it in politician's bag (9)
I believe the answer is:
territory
'bag' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'slip it in politician's' is the wordplay.
'slip' becomes 'err' (to err is to make an error).
'in' is an insertion indicator.
'politician' becomes 'tory' (term for a Conservative Party politician).
'err'+'it'='errit'
'errit' put within 'tory' is 'TERRITORY'.
Can you help me to learn more?
(Other definitions for territory that I've seen before include "province" , "Try rioter (anag.)" , "Region marked off for some purpose" , "ground" , "All the land belonging to a state" .)